Synthesis technology of lithium iron phosphate

　　The most studied cathode materials are LiCoO2, LiNiO2, LiMn2O4 and their derivatives, such as LiNi0.8Co0.2O2, LiNil/3Co1/3Mnl/302, etc.

　　LiCoO2 is the only large-scale commercialized cathode material. Currently, more than 90% of commercial lithium-ion batteries use LiCoO2 as the cathode material. The research on LiCoO2 is relatively mature and has excellent overall performance, but it is expensive, has low capacity, is highly toxic, and has certain safety issues.

　　LiNiO2 has lower cost and higher capacity, but it is difficult to prepare, has poor consistency and reproducibility of material properties, and has serious safety issues.

　　LiNil/3Co1/3Mnl/302 can be regarded as a solid solution of LiNi02 and LiCoO2. It has the advantages of both LiNiO2 and LiCoO2. It was once considered to be the most likely new cathode material to replace LiCoO2. However, the synthesis conditions are still relatively harsh (oxygen atmosphere is required). ), poor safety and other shortcomings, and the overall performance needs to be improved; at the same time, because it contains more expensive Co, the cost is also higher.

　　Spinel LiMn2O4 has low cost and good safety, but has poor cycle performance, especially high-temperature cycle performance, has a certain solubility in the electrolyte, and has poor storage performance.

　　The new ternary composite oxide lithium nickel cobalt manganate (LiNil/3Co1/3Mnl/302) material combines the respective advantages of LiCoO2, LiNiO2, LiMn2O4 and other materials: the cost is equivalent to LiNil/3Co1/3Mnl/3O2, and the reversible capacity is large. The structure is stable and safe, between LiNi0.8Co0.2O2 and LiMn2O4. It has good cycle performance and is easy to synthesize; but because it contains more expensive Co, the cost is also higher.

　　For lithium-ion batteries with medium to large capacity and medium to high power, the cost, high-temperature performance, and safety of the cathode material are very important. The above-mentioned LiCoO2, LiNiO2, LiMn2O4 and their derivative cathode materials cannot yet meet the requirements. Therefore, research and development of new cathode materials that can be used in lithium-ion batteries with medium to large capacity and medium to high power have become a current hot topic.

　　The orthorhombic olivine structure LiFePO4 cathode material has gradually become a new research hotspot at home and abroad. Preliminary research shows that this new cathode material combines the respective advantages of LiCoO2, LiNiO2, LiMn2O4 and their derivative cathode materials: it does not contain precious elements, the raw materials are cheap, and the resources are extremely abundant; the working voltage is moderate (3.4V); the platform characteristics are good, The voltage is extremely stable (comparable to a regulated power supply); the theoretical capacity is large (170mAh/g); the structure is stable and the safety performance is excellent (O and P are firmly combined with strong covalent bonds, making it difficult for the material to undergo oxygen evolution and decomposition); high temperature The performance and thermal stability are significantly better than other known cathode materials; the cycle performance is good; the volume is reduced during charging, and the volume effect is good when combined with carbon negative electrode materials; the compatibility with most electrolyte systems is good, and the storage performance is good; Non-toxic, truly green material. Compared with LiCoO2, LiNiO2, LiMn2O4 and their derivative cathode materials, LiFePO4 cathode material has outstanding advantages in cost, high temperature performance and safety, and is expected to become the preferred cathode material for medium to large capacity and medium to high power lithium-ion batteries. The industrialization and popularization of this material are of great significance to reducing the cost of lithium-ion batteries, improving battery safety, expanding the lithium-ion battery industry, and promoting the large-scale and high-power lithium-ion batteries. It will make lithium-ion batteries more popular in medium and large Applications in capacity UPS, medium and large energy storage batteries, power tools, and electric vehicles have become a reality.

　　However, lithium iron phosphate has two obvious shortcomings. One is low conductivity, which leads to poor high-rate charge and discharge performance and low actual specific capacity; second, low packing density leads to low volume specific capacity. These two shortcomings hinder the practical application of this material.

　　Currently, people's research attention is focused on solving the problem of low conductivity of lithium phosphate, and significant progress has been made. The improvement measures taken mainly include:

　　(1) Incorporate conductive carbon materials or conductive metal particles into lithium iron phosphate particles, or coat the surface of lithium iron phosphate particles with conductive carbon materials to increase the electronic conductivity of the material.

　　(2) Dope a small amount of impurity metal ions, such as Mg2+, Ti4+, Zr4+, Nb5+, into the lithium iron phosphate (1iPePO4) lattice to replace part of the Li+ wide positions, thereby transforming the intrinsic semiconductor of lithium iron phosphate into n-type or p type semiconductor, significantly improving the electronic conductivity of the material.

　　(3) Add impurity elements such as Mn2+ into lithium iron phosphate to replace part of the Fe2+ position, increase the unit cell parameters of lithium iron phosphate, and improve the lithium ion conductivity of the material.

　　(4) Use new processes such as sol-gel method and liquid phase synthesis method to reduce the size of lithium iron phosphate grains, and even synthesize nano-lithium iron phosphate to shorten the diffusion distance of Li+ as much as possible and apparently increase the lithium ion content of the material. Conductivity and material utilization.

　　However, the shortcomings of low packing density of lithium iron phosphate have been ignored and avoided by people, and have not yet been solved, hindering the practical application of the material. The theoretical density of lithium cobalt oxide is 5.1g/cm3, and the tap density of commercial lithium cobalt oxide is generally 2.0-2.4g/cm3; while the theoretical density of lithium iron phosphate is only 3.6g/cm3, which is inherently higher than lithium cobalt oxide. much lower. In order to improve the conductivity, people incorporate conductive carbon materials, which also significantly reduces the packing density of the material, making the general tap density of carbon-doped lithium iron phosphate only 1.0-1.2g/cm3. Such a low packing density makes the volume specific capacity of lithium iron phosphate much lower than that of lithium cobalt oxide. The resulting battery will be very large, which not only has no advantages at all, but is also difficult to apply in practice.

　　Therefore, improving the packing density and volume specific capacity of lithium iron phosphate is of decisive significance for the practical application of lithium iron phosphate. The particle morphology, particle size and distribution of powder materials directly affect the packing density of the material. For example, Ni(OH)2 is the cathode material used in nickel-metal hydride batteries and nickel-cadmium batteries. In the past, people used flake Ni(OH)2, whose tap density was only 1.5-1.6g/cm3;

　　The tap density of spherical Ni(OH)2 currently used can reach 2.2-2.3g/cm3; spherical Ni(OH)2 has basically replaced flake Ni(OH)2, significantly improving the performance of nickel-metal hydride batteries and nickel-metal hydride batteries. Energy density of cadmium batteries. Drawing on the research results of high-density spherical Ni(OH)2, our laboratory has successfully developed a series of high-density spherical cathode materials for lithium-ion batteries, including LiCoO2, liMn2O4, LiNi0.8Co0.2O2, LiNil/3Co1/3Mnl/3O2, etc. Among them, the tap density of LiCoO2 and LiNi0.8Co0.2O2 can reach 2.9g/cm3, which is much higher than that of similar commercial materials. Research and practical applications have shown that spherical products not only have outstanding advantages such as high packing density and large volume specific capacity, but also have excellent fluidity, dispersion and processability, which are very conducive to the production of cathode material slurries and electrode sheets. coating to improve the quality of electrode sheets; in addition, compared to irregular particles, the surface of regular spherical particles is easier to coat with a complete, uniform and strong modification layer, so spherical products are more likely to further improve their overall performance through surface modification.

　　On this basis, we propose that spheroidization is the development direction of cathode materials for lithium-ion batteries. Currently reported at home and abroad. LiFePO4 cathode materials are composed of irregular particles, and the packing density and energy density of powder materials are low. Therefore, this project is dedicated to the spheroidization of LiFePO4 material particles. Through the spheroidization of particles, the packing density and volume specific capacity of the material can be improved. On this basis, the advantage of spherical materials that are easy to be surface coated will be taken into account, and the surface coating of spherical particles can be further improved. Modification improves the overall performance of the material. In the process of spheroidizing and surface modification of LiFePO4 material particles, we fully learn from, absorb, and utilize the excellent results that people have achieved in improving the conductivity of lithium iron phosphate; finally, we prepare spherical, high packing density, and high volume specific capacity , high conductivity LiFePO4 cathode material, enabling it to be used in medium-to-large capacity, medium-to-high power lithium-ion batteries, and promoting the industrialization of this material.

　　At present, our research laboratory uses divalent iron salts or trivalent iron salts, phosphoric acid or phosphate, and ammonia water as raw materials to synthesize high-density spherical iron phosphate precursors through controlled crystallization technology, and then blends them with lithium sources and carbon sources for heat treatment. Carbon-doped high-density spherical lithium iron phosphate was synthesized by carbothermal reduction method. The lithium iron phosphate powder material is composed of monodisperse spherical particles with a particle size of 5-10um, a large packing density (tap density up to -1.8g/cm3), good fluidity, good processability, and a reversible capacity of 140MLNg.

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